Nanosize structures composed of valve metals and valve metal suboxides and process for producing them

ABSTRACT

A strip-like or sheet-like valve metal or valve metal suboxide structure which has a transverse dimension of from 5 to 100 nm.

The present invention relates to novel lamellar structures of valve metals and valve metal suboxides which have a dimension of less than 100 mm in one direction and a process for producing them.

Fine structures composed of metals and metal suboxides which are present in powders or surface regions of larger metal substrates have a wide variety of uses as catalysts, support materials for catalysts, in the field of membrane and filter technology, in the medical sector as implant material, as storage materials in secondary batteries and as anode material of capacitors because of their large specific surface area.

WO 00/67936 discloses a process for producing finely divided valve metal powders by reduction of valve metal oxide powders by means of gaseous reducing metals such as Mg, Al, Ca, Li and Ba. Owing to the volume shrinkage in the reduction of the oxide to the metal and the volume increase caused by the solid oxides of the reducing metals which are formed, highly porous valve metal powders having a high specific surface area which are suitable, in particular, for producing solid electrolyte capacitors are formed.

It has now been found that under particular reduction conditions lamellar structures having transverse dimensions in the nanometre range are formed, with the laminates initially comprising alternate layers of the reduced valve metal oxide and of the oxidized reducing metal.

Dissolution and leaching of the oxide of the reducing metal in mineral acids enables the nanosize valve metal structures to be freed of the oxide of the reducing metal.

Depending on the geometric structure of the starting valve metal oxide, finely divided powders having a lamellar structure or strip-like or lamellar surface structures on metal substrates having relatively coarse/large structures are obtained, with the metal and/or suboxide strips or lamellae having a width of less than 100 nm and a spacing (intermediate space) which can be up to twice the strip width, depending on the valve metal oxide and the oxidation state which it attains.

Thus, when finely divided valve metal oxide powders having average dimensions of the primary structure particle size of from 50 to 2000 nm, preferably less than 500 nm, more preferably less than 300 nm, are used, finely divided metal or suboxide powders having a lamellar structure and a width of the metal or suboxide strips of from 5 to 100 nm, preferably from 8 to 50 nm, particularly preferably up to 30 nm, and transverse dimensions of from 40 to 500 nm and a specific surface area of above 20 m²/g, preferably above 50 m²/g, are obtained.

When relatively large valve metal oxide substrates having dimensions above, for example, 10 μm are used, metallic or suboxidic strips having a width of up to 100 nm, preferably from 5 to 80 nm, particularly preferably from 8 to 50 nm, more preferably up to 30 nm, and spacings of from one to two times the strip width are obtained on these structures. The depth of the grooves between the strips can be up to 1 μm.

Relatively large metal structures or substrates, for example wires or foils, having a strip-like surface can be obtained by firstly oxidizing the surface chemically or anodically and then reducing the surface according to the invention, with the strip depth being determined by the thickness of the oxide layer initially produced.

Furthermore, structures according to the invention can be obtained by providing a substrate comprising, for example, another metal or ceramic with a valve metal oxide layer, for example by application of a valve metal layer by vapour deposition or electrolytic deposition, oxidizing the coating and reducing it according to the invention to the metal or suboxide.

Valve metal oxides used for the purposes of the present invention can be oxides of the elements of transition groups 4 to 6 of the Periodic Table, e.g. Ti, Zr, V, Nb, Ta, Mo, W and Hf, and also their alloys (mixed oxides) and Al, preferably Ti, Zr, Nb and Ta, particularly preferably Nb and Ta. As starting oxides, preference is given to, in particular, Nb₂O₅, NbO₂ and Ta₂O₅. Preferred reaction products according to the invention are the metals of the starting oxides. Lower oxides (suboxides) of the starting valve metal oxides can also be obtained as reduction products. A particularly preferred reduction product is niobium suboxide having metallically conducting properties of the formula NbO_(x) where 0.7<x<1.3, which, in addition to tantalum and niobium, is suitable as anode material for capacitors, according to the invention particularly for use in the range of low activation voltages up to 10 V, particularly preferably up to 5 V, in particular up to 3 V.

As reducing metals, it is possible to use Li, Mg, Ca, B, and/or Al and their alloys according to the invention. Preference is given to Mg, Ca and Al, as long as these are less noble than the metals of the starting oxides. Very particular preference is given to Mg or a eutectic of Mg and Al.

A characteristic of the reduction products according to the invention is their content of reducing metals in the range above 10 ppm, in particular from 50 to 500 ppm, owing to doping during reduction.

The process of the invention by which the nanosize structures can be produced is based on reduction of metal oxides by reducing metals in vapour form as described in WO 00/67936. Here, the valve metal oxide to be reduced in powder form is brought into contact with the vapour of the reducing metal in a reactor. The reducing metal is vaporized and conveyed by means of a carrier gas stream such as argon over the valve metal oxide powder present on a mesh or in a boat at elevated temperature, typically from 900 to 1200° C., likewise typically for a period of from 30 minutes to some hours. Since the molar volume of valve metal oxides is from two to three times the volume of the corresponding valve metal, a considerable decrease in volume takes place during reduction. Sponge-like, highly porous structures in which the oxide of the reducing metal is deposited are therefore formed in the reduction. Since the molar volumes of the oxides of the reducing metals are greater than the difference between the molar volumes of the valve metal oxide and the valve metal, they are incorporated into the pores with production of residual stresses. The structures can be freed of the oxides of the reducing metals by dissolution of these oxides, so as to obtain highly porous metal powders. Studies on the mechanism of the reduction and the formation of the pores and their distribution have shown the following: starting from small reaction nuclei on the surface of the valve metal oxide particles or substrates, layer-like structures having nanosize dimensions are formed behind the valve metal/valve metal oxide reaction front in the initial phase of the reaction. The layers are firstly oriented perpendicular to the surface in regions of the particles/substrates close to the surface. However, as the reaction front moves deeper into the oxide particles/substrate, orientation and dimensions of the lamellae are determined by the crystal orientation and dimensions of the primary particles in the valve metal oxide and by the reaction conditions. A certain number of lattice planes in a valve metal oxide crystallite are replaced by a stoichiometrically equivalent number of lattice planes of the valve metal and of the oxide of the reducing metal. These nanosize layer structures, which are actually very energetically unfavourable because of the high interfacial stress, are nevertheless produced and become possible since the reduction is strongly exothermic and at least part of the excess energy is not dissipated as heat but “invested” in structure formation which makes fast reaction kinetics possible. The many flat interfaces of the layer structures act as “fast roads” for the atoms of the reducing metals, i.e. they allow fast diffusion and thus reaction kinetics which lead quickly and effectively to reduction of the total energy of the reaction system. However, the layer-like structures composed of valve metals and oxides of the reducing metals are formed only in a metastable state which on introduction of thermal energy leads to a structural state having an even lower energy. In a reduction process carried out “normally” with relatively long heat treatment times and constant reaction conditions (temperature, vapour pressure of the reducing metal, etc.), this structural transformation inevitably occurs, i.e. the nanosize layer structures are converted into a greatly coarsened and interpenetrating structure composed of valve metal regions and regions of the reduction metal oxides.

It has now been found that the lamellar structures can be frozen if care is taken to ensure that the reduction product is cooled to a temperature at which the lamellar structures remain stable before transformation of the structures occurs. According to the invention, the reduction conditions are therefore set so that the reduction can proceed very uniformly within a short time, i.e. if a pulverulent starting oxide is used, within the powder bed of the oxide and the reduction product is cooled as quickly as possible immediately after the reduction is complete.

For this reason, preference is given to employing a low thickness of the powder bed to ensure uniform permeation of the vapour of the reducing metal through the bed. The thickness of the powder bed is particularly preferably less than 1 cm, more preferably less than 0.5 cm.

Furthermore, the uniform permeation of the vapour of the reducing metal through the powder bed can be ensured by providing for a large free path length of the vapour of the reducing metal. According to the invention, the reduction is therefore preferably carried out under reduced pressure, more preferably in the absence of carrier gases. The reduction is particularly preferably carried out at a vapour pressure of the reducing metal of from 10⁻² to 0.4 bar, more preferably from 0.1 to 0.3 bar, in the absence of oxygen. A low carrier gas pressure of up to 0.2 bar, preferably less than 0.1 bar, can be accepted without disadvantages. Suitable carrier gases are, in particular, noble gases such as argon and helium and/or hydrogen.

The increase in depth of the lamellar structures decreases with increasing depth as a result of the longer diffusion path along the interface between reduced metallic lamellae and the oxide of the reducing metal formed between the metallic lamellae. It has been found that essentially no transformation of the lamellar structure takes place during the reduction up to a depth in the material of up to 1 μm.

Preference is therefore given, according to the invention, to using valve metal oxide powders whose smallest cross-sectional dimension of the primary structure particle size (crystallite dimension) does not exceed 2 μm, preferably 1 μm, particularly preferably an average of 0.5 μm. The valve metal oxide powders can be used as porous sintered agglomerates if the primary structures have appropriately small dimensions. It is also advantageous for the primary particles to be sintered together strongly but a hierarchically structured network of open pores to be present between the agglomerated primary particles, so that the pore size distribution of the open pores makes it possible for the vapour of the reducing metal to directly reach and reduce a very large proportion of the surfaces of the primary particles.

Even though they are significantly less effective than the pore channels, grain boundaries between adjacent primary particles can also accelerate diffusion. It is therefore advantageous for very high proportions of grain boundaries between the primary particles to be formed in addition to small primary particles and an open porosity in the aggregated valve metal oxide particles. This is achieved by optimization of the primary particle size and sintering in the precipitation of oxide precursors as hydroxides and the calcination of the hydroxides to form the valve metal oxides. Calcination is preferably carried out at temperatures of from 400 to 700° C. The calcination temperatures are particularly preferably from 500 to 600° C.

In the production of metal foils or wires having a lamellar surface structure, preference is given to using metal foils or wires whose surface has an oxide layer having a thickness of less than 1 μm, preferably less than 0.5 μm.

After the reduction under subatmospheric pressure, which can take from a few minutes to some hours, preferably from about 10 to 90 minutes, depending on the reducing metal vapour or metal vapour mixture used and its vapour pressure, the reduction is stopped by interrupting the supply of the vapour of the reducing metal and the reduced valve metal is quickly cooled to a temperature below 100° C. in order to stabilize the nanosize lamellar structure of layers of valve metal or valve metal suboxide and oxide of the reducing metal. Sintering of adjacent lamellar structures having different orientations with slight coarsening can be accepted. Cooling can be brought about, for example, by means of a fast pressure increase by introduction of protective gas (cooling gas), preferably argon or helium. Preference is given to cooling to 300° C. within 3 minutes, further to 200° C. within a further 3 minutes and further to 100° C. within a further 5 minutes.

According to the invention, the reduction is preferably carried out at a comparatively low temperature to minimize coarsening of the nanosize lamellar structures. A temperature of the valve metal oxide to be reduced of from 500 to 850° C., more preferably less than 750° C., particularly preferably less than 650° C., is preferred. Here, the actual temperature can be considerably exceeded at the beginning of the reduction because of the exothermic nature of the reduction reaction.

The various measures according to the invention for avoiding disintegration and coarsening of the nanosize lamellar structures composed of reaction product and oxidized reducing metal which are initially formed in the reduction can be used as alternatives or in combination.

For example, at a high reduction temperature it is sufficient to ensure a short reduction time by providing for effective, fast access of the vapour of the reducing metal, for example by means of a small powder bed of the starting metal oxide and/or a reduced carrier gas pressure, i.e. an increased free path length for the atoms of the vapour of the reducing metal.

On the other hand, at a low reduction temperature longer reduction times can be accepted.

Starting valve metal oxide powder agglomerates having an advantageous open-pore structure require less stringent process conditions to achieve the lamellar structure according to the invention.

After the reduction is complete and the reduced valve metal oxide has been cooled and made inert by gradual introduction of oxygen or air, the enclosed oxide of the reducing metal can be leached from the resulting nanosize structure, for example by means of mineral acids such as sulphuric acid or hydrochloric acid or mixtures thereof, washed with demineralized water until neutral and dried.

In the case of the reduction of finely divided powders, these comprise particles having a tabular primary structure which are partly grown into one another in a dendrite-like fashion.

After the oxides of the reducing metals have been leached out, the now free-standing lamellar structures of the valve metals remain geometrically stable since they are sufficiently well sintered to the adjacent, generally differently oriented lamellar structures via the end parts of the individual layers. The original (polycrystalline) valve metal oxide particle has thus been converted into an aggregated valve metal particle whose primary particles comprise layer structure groups of differing orientation and which are sintered to one another. Overall, a stable interpenetrating structure of metal and “flat” pores has thus been formed.

FIG. 1 schematically shows an apparatus for carrying out the process of the invention. The reactor which is generally denoted by 1 has a reduction chamber 2. Reference numeral 3 denotes the temperature control which comprises heating coils and cooling coils. Protective or flushing gas or cooling gas is introduced via a valve into the reduction chamber in the direction of the arrow 4. The reduction chamber is evacuated or gases are taken off in the direction of the arrow 5. The reduction chamber 2 is joined by a vaporization chamber 6 for the reducing metal which is provided with separate heating 7. The thermal separation of vaporization chamber and reduction chamber is effected by means of the valve region 8. The valve metal oxide to be reduced is present as a thin powder bed in the boat 10. If valve metal oxide foils or wires or foils or wires having a surface composed of valve metal oxide are used, these are preferably suspended vertically and parallel to the flow of the vapour of the reducing metal in the reduction chamber. The reducing metal in the boat 9 is heated to a temperature which provides the desired vapour pressure.

The oxide powder is introduced as a bed having a height of 5 mm in a boat. A boat containing magnesium turnings is placed in the vaporization chamber. The reactor is flushed with argon. The reduction chamber is then heated to the reduction temperature and evacuated to a pressure of 0.1 bar. The vaporization chamber is subsequently heated to 800° C. The magnesium vapour pressure (static) is about 0.04 bar. After 30 minutes, the heating of reduction chamber and vaporization chamber is switched off and argon which has been cooled by depressurization from 200 bar is introduced and passed through the reduction chamber for a further period. The reduction chamber walls are at the same time cooled by means of water.

FIGS. 2, 3 and 4 show transmission electron micrographs of tantalum powder which has been reduced according to the invention after focused ion beam preparation of the reduction product at various magnifications. The dark stripes in the figures are tantalum lamellae and the light-coloured stripes are magnesium oxide lamellae. The different orientations of the lamellar structures correspond to different crystallite orientations of the starting tantalum pentoxide. 

1.-14. (canceled)
 15. A strip-like or sheet-like valve metal or valve metal suboxide structure which comprises a transverse dimension of from 5 to 100 nm.
 16. The valve metal or valve metal suboxide structure according to claim 15 having a sheet- or layer-like primary structure in the form of powders.
 17. The valve metal or valve metal suboxide structure according to claim 15 in the form of a surface strip structure.
 18. The valve metal or valve metal suboxide structure according to claim 17 in the form of foils or wires having strips having a width of from 5 to 100 nm and a strip spacing of from one to 2 times the strip width.
 19. The valve metal or valve metal suboxide structure according to claim 15, wherein the strips or lamellae are aligned parallel in groups.
 20. The valve metal or valve metal suboxide structure according to claim 15, wherein the transverse dimension or strip width is from 8 to 50 nm.
 21. The valve metal or valve metal suboxide structure according to claim 15 comprising Ti, Zr, V, Nb, Ta, Mo, W, Hf or Al or alloys thereof.
 22. The valve metal or valve metal suboxide structure according to claim 15 comprising Nb or Ta or alloys thereof.
 23. The valve metal suboxide structure according to claim 15 having the formula NbO_(x) where 0.7<x<1.3.
 24. The valve metal or valve metal suboxide structure according to claim 15, having a content of at least one reducing metal in an amount of from 10 to 500 ppm.
 25. A process for the reduction of the valve metal oxides by means of the vapor of reducing metals at a temperature sufficient for reduction to form lamellar nanosize structures, wherein the reduced valve metal oxide is frozen before thermal decomposition of the lamellar structure and transformation into coarsened structures.
 26. The process according to claim 25, wherein the reduction is carried out at an inert gas pressure of less than 0.2 bar and a vapor pressure of the reducing metal of from 10⁻² to 0.4 bar.
 27. The process according to claim 25, wherein the reduction product is cooled to below 100° C. within a few minutes immediately after the reduction is complete.
 28. The process according to claim 25, wherein the reducing metal is Li, Al, Mg or Ca or a mixture thereof.
 29. The process according to claim 25, wherein the reducing metal is Mg.
 30. The process according to claim 25, wherein oxides of Al, Hf, Ti, Zr, V, Nb, Ta, Mo and/or W or mixed oxides thereof, are used as oxides to be reduced.
 31. The process according to claim 25, wherein oxides of Nb or Ta or mixed oxides thereof are used as oxides to be reduced. 